The present invention relates a motorized power steering apparatus for power assisting the steering of a motor vehicle through the rotating force of a motor.
Conventionally, in this type of power steering apparatus, for the purpose of detecting a steering torque, a steering-torque to displacement converter is provided for converting the steering torque applied to a steering wheel by a driver into a twisting displacement of a torsion bar. A potentiometer is mounted on the converter so that it converts the displacement output of the converter into an electric signal with one half of the source voltage for the potentiometer being made as the neutral point for the steering torque.
In order to ensure fail-safe operation of the apparatus in the case of a failure in the potentiometer, it is a general trend to employ another potentiometer, so that the output signal of one of the two potentiometers is utilized as a primary signal for controlling the apparatus. The output signal of the other potentiometer is used as an auxiliary signal for comparison with the primary signal. When the difference between the primary signal and the auxiliary signal is greater than a prescribed level, a control unit determines that the potentiometers are abnormal. It thereafter performs a fail-safe operation whereby a fail-safe relay is turned off to deenergize a power steering motor and an electromagnetic clutch which controls transmission of a power steering torque from the motor toward steerable road wheels.
With the above-mentioned conventional apparatus, however, the power source for the potentiometers, which act as a torque sensor, is a constant voltage power source which is commonly utilized as a power supply to other electronic elements such as a microcomputer incorporated in the control unit. Accordingly, from an economical point of view, it is general practice to supply power to the primary and auxiliary potentiometers from the common power source through a buffer.
As a result, if a failure takes place in which the source voltages for the primary and auxiliary potentiometers concurrently drop for some reason, the voltage at the neutral point of each potentiometer falls or rises. This causes the primary potentiometer to generate an erroneous signal indicative of a steering torque being applied to the steering wheel by the driver even if there is actually no steering torque applied.
In this kind of failure, however, the output signals of the primary and auxiliary potentiometers concurrently fall or rise, so there will be no appreciable difference therebetween. As a result, the control unit determines that there are no abnormalities in the potentiometers, and hence it does not perform any fail-safe operation, allowing an erroneous and improper power steering operation due to the motor.